A generator (4) is combined with drive wheels (8) of a vehicle, and the output of an engine (2) is transmitted to the drive wheels (8) under an engaging force of an electromagnetic clutch (3) in response to an energizing current. During a predetermined deceleration state, a braking force is applied to the drive wheels (8) and a battery (15) is charged by the generator (4) which performs regenerative power generation. At this time, the controller (16) supplies a weak energizing current to the electromagnetic clutch (3) so as to put the clutch (3) into a partially engaged state. A sensor (29) detects the output rotation speed of the clutch (3). When the output rotation speed of the clutch (3) is high, the controller (16) decreases the energizing current so that the rotational resistance of the electromagnetic clutch (3) is reduced. When the output rotation speed of the clutch (3) is low, the controller (16) increases the energizing current so that the re-engaging response of the electromagnetic clutch (3) is increased.